Furnace

ABSTRACT

A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.

FIELD

The present disclosure generally relates to a furnace, and moreparticularly to a furnace for heating an interior of an enclosure.

BACKGROUND

Furnaces or heaters are commonly used to heat fluid, such as air,circulated through a building to heat its interior. Some heaters burnsolid fuel, such as wood or coal. The heaters generally include afirebox in which the fuel is burned. Air is circulated to and from theheater via a duct system generally including a cold air duct and a hotair duct communicating with the building interior. The furnace receivesair from the building interior via the cold air duct. The air is heatedas it flows over the firebox. The heated air is returned to the buildinginterior via the hot air duct to heat the building interior.

Although conventional furnaces of this type work to heat the buildinginterior, the furnaces may suffer from inefficiency in burning the fuel,inefficiency in transferring heat from the products of combustion to thebuilding interior, and high emissions of undesirable combustionby-products. Furnaces are commonly used for many years and can requiremaintenance and repair for long term durability and desired emissionsperformance. For example, furnaces with electronic controls can requiremaintenance to update or replace electronic components. Moreover, in theevent of a power outage, the electronic control may become inoperable.Some furnaces may use a catalytic emissions reduction system. Suchcatalytic systems are prone to blockage and usually do not operateefficiently at low temperatures.

SUMMARY

One aspect of the present disclosure relates to a forced-air furnace forheating a space. The furnace includes a housing having a top, bottom,front, rear, and opposite sides. A firebox in the housing has acombustion chamber adapted for receiving fuel to be combusted forproducing products of combustion. The furnace includes a combustion airdelivery system for delivering combustion air to the combustion chamber.The combustion air delivery system includes a primary combustion airpassage including a primary combustion air outlet in the combustionchamber for delivering primary combustion air to the combustion chamber.The combustion air delivery system includes a secondary combustion airpassage including a secondary combustion air outlet positioned in thecombustion chamber for delivering secondary combustion air to thecombustion chamber. The combustion air delivery system includes a valvesystem in fluid communication with the secondary combustion air passageconfigured for changing the amount of secondary combustion air deliveredto the combustion chamber in response to combustion chamber temperature.

In another aspect of the disclosure, a forced-air furnace for heating aspace includes a housing having a top, bottom, front, rear, and oppositesides. A firebox in the housing has a combustion chamber adapted forreceiving fuel to be combusted for producing products of combustion. Aheat transfer device is above the firebox. A forced-air system includesa blower configured for moving air to the heat transfer device. The heattransfer device includes a post-combustion plenum having an inlet influid communication with the combustion chamber for receiving productsof combustion therefrom and an exit for permitting products ofcombustion to exit the post combustion plenum. The post-combustionplenum has a first side, a second side opposite the first side, and alength extending therebetween. The heat transfer device includes heattransfer passaging downstream from the blower for receiving air from theblower to be heated by the post-combustion plenum. The heat transferpassaging includes at least one first passage portion extendinglengthwise along the post-combustion plenum defining a flow pathextending in a direction toward the first side of the post-combustionplenum. The heat transfer passaging includes a second passage portiondownstream from the first passage portion. The second passage portionextends lengthwise along the post-combustion plenum and defines a flowpath extending in a direction toward the second side of thepost-combustion plenum.

In another aspect of the disclosure, a forced-air furnace for heating aspace includes a housing having a top, bottom, front, rear, and oppositesides. A firebox in the housing has a combustion chamber adapted forreceiving fuel to be combusted for producing products of combustion. Thecombustion chamber has an exit for permitting products of combustion toexit the combustion chamber. A combustion air delivery system isprovided for delivering combustion air to the combustion chamber. Thecombustion air delivery system includes a primary combustion air passageincluding at least one primary combustion air outlet in the combustionchamber for delivering primary combustion air to the combustion chamber.The combustion air delivery system includes a secondary combustion airpassage including a secondary combustion air outlet positioned in thecombustion chamber for delivering secondary combustion air to thecombustion chamber. The combustion chamber exit is positioned adjacent afirst end of the combustion chamber. The primary combustion air outletis positioned adjacent the first end of the combustion chamber lowerthan the combustion chamber exit. The primary combustion air outlet isconfigured for directing primary combustion air therefrom toward asecond end of the combustion chamber opposite the first end.

Other objects and features of the present disclosure will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective of a furnace as described;

FIG. 2 is a rear perspective of the furnace of FIG. 1;

FIG. 3 is a front perspective of the furnace partially separated;

FIG. 4 is a rear perspective of the furnace partially separated;

FIG. 5 is a front perspective of components of a combustion air deliverysystem of the furnace;

FIG. 6 is a front perspective of a valve of the combustion air deliverysystem of FIG. 5;

FIG. 7 is a cross section of the furnace taken in a plane including line7-7 of FIG. 1;

FIG. 8 is an enlarged fragmentary view of the section of FIG. 7, a valvemember of the valve being shown in a closed position;

FIG. 9 is an enlarged fragmentary view similar to FIG. 8 but showing thevalve member in an open position;

FIG. 10 is a cross section of the furnace taken in a plane includingline 10-10 of FIG. 1;

FIG. 11 is a front perspective of the furnace having portions of afurnace housing broken away to show internal components;

FIG. 12 is a cross section of the furnace taken in a plane includingline 12-12 of FIG. 1; and

FIG. 13 is graph illustrating an example curve of flow area through thevalve as a function of temperature.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a furnace or heater according to the presentdisclosure is designated generally by the reference number 20. Thefurnace 20 may be used to heat air (broadly, fluid) that is circulatedin a space such as an interior of a building (not shown) to heat thespace. The illustrated furnace 20 is an indoor, forced-air furnaceconfigured for burning fuel such as wood. As described in further detailbelow, the furnace 20 automatically heats air and circulates the air toheat the interior of the building. As will be appreciated by thoseskilled in the art, aspects of the present disclosure can be adapted foruse in other types of furnaces. For example, aspects of the disclosurecan be used for outdoor furnaces, furnaces that burn other types offuel, and furnaces that heat fluid other than air.

As will become apparent, the furnace has several features that improvethe efficiency of the furnace and reduce harmful combustion by-productemissions. In general, the furnace is designed to achieve a relativelyhigh efficiency and substantially complete burn of the fuel. After thecomplete burn, heat is efficiently transferred from the products ofcombustion to air to be delivered to the space to be heated. These andother aspects of the furnace will be described in further detail below.

Referring to FIGS. 1 and 2, the furnace 20 comprises a housing generallyindicated at 22. The housing 22 includes a top defined in part by anupper wall 24 and a bottom defined in part by a lower wall 26 from whichfour legs extend to support the furnace 20 above a surface. The housing22 includes a front defined in part by a front wall 28. The housing 22has a back defined in part by a rear wall 30, a primary combustion airsupply plenum 32 (FIG. 4), a blower housing 34, and a control housing35. The housing 22 has left and right sides defined in part by left andright side walls 36, 38. The furnace walls are made of a suitablematerial such as heavy gauge steel and may be thermally insulated.Housings having other configurations and shapes can be used.

Referring to FIG. 3, the housing 22 has a hollow interior that houses afirebox generally designated by 40 in which fuel is burned, and a heattransfer device generally designated by 42 for transferring heat fromproducts of combustion to the air delivered to the space to be heated.As explained in further detail below, a draft blower 44 deliverscombustion air to the fire box 40, and a forced-air blower 46 within theblower housing 34 moves air through the heat transfer device 42 to heatthe air for delivery to the space to be heated.

As shown in FIGS. 7 and 10, the firebox 40 includes a combustion chamber50 and an ash chamber or receptacle 52 below the combustion chamber. Asuitable fuel support 54 (e.g., a grate) is provided in the combustionchamber 50 above the ash chamber 52 for supporting fuel such as wood inthe combustion chamber. The combustion chamber 50 has a front adjacentthe front of the housing 22, left and right sides adjacent the left andright sides of the housing, and a rear adjacent the rear of the housing.Fuel can be loaded into the combustion chamber 50 through a frontopening that is normally closed by a fuel door 56. The combustionchamber 50 is sized to hold about 3.5 cubic feet of fuel (4.5 cubic feetof fuel in a larger version) surrounded by air for burning. It will beunderstood that the furnace with this combustion chamber capacity issuitable for heating a relatively large volume of space or spaces, suchas the interior of several rooms, or an entire home or other building.Other sizes can be used. Ashes may be removed from the combustionchamber through an ash opening that is normally closed by an ash door58. The combustion chamber 50 may be lined with fire brick or similarprotective panels.

Fuel burning within the combustion chamber 50 is fed by oxygen deliveredby a combustion air delivery system generally designated by 60 (FIG. 5),which will be described with reference to FIGS. 4-10. The combustion airdelivery system 60 includes the draft blower 44, the primary combustionair supply plenum 32, and primary and secondary air passages fordelivering primary and secondary combustion air to the combustionchamber, generally indicated by 62 and 64, respectively (FIG. 5). Aswill become apparent, the primary combustion air from the primary airpassage 62 feeds primary combustion of the fuel, and the secondarycombustion air from the secondary air passage 64 feeds secondarycombustion for a cleaner, more complete burn.

The draft blower 44 is mounted on the primary combustion air supplyplenum 32 such that an outlet of the blower overlies an inlet 66 of theplenum (FIG. 5). The plenum inlet 66 is a circular opening having a flowarea of about 4.9 square inches (about 2.5 inches in diameter). In oneexample, the draft blower 44 may be designed to provide about 50 CFM ofair flow. The draft blower 44 may be thermostatically controlled tooperate in a forced draft mode and a natural draft mode, toautomatically generate desired heat, as explained in further detailbelow. The primary combustion air supply plenum 32 has an L shape. Inthe illustrated embodiment, the primary combustion air supply plenum 32is bounded by the rear wall 30 and an L-shaped housing portion mountedon the rear wall. Referring to FIG. 5, the plenum 32 includes an upperfirst portion 32A and a lower second portion 32B. The first portion 32Aextends downward to the lower portion 32B, and the lower portion extendslaterally toward opposite sides for feeding combustion air to theprimary combustion air passage 62.

Still referring to FIG. 5, the primary combustion air passage 62includes two ducts 72 on opposite sides of the firebox 40 and a frontduct 74 adjacent the front of the combustion chamber 50 below the fuelopening. The side ducts 72 communicate with the lower portion of thecombustion air supply plenum 32B and extend forward from opposite endsthe plenum toward the front of the firebox 40 to opposite ends of thefront duct 74. The side ducts 72 are connected to opposite ends of thefront duct 74, which has a primary combustion air outlet 76. The ducts72, 74 are exposed inside the combustion chamber 50 to preheat primarycombustion air in the ducts. In one example, the ducts 72, 74 may be 2inch square tubing having an internal flow area of about 3.5 squareinches (1⅞ inches by 1⅞ inches). Primary combustion air traveling in theprimary combustion air passage 62 may be preheated to about 300 degreesF. before reaching the primary combustion air outlet 76. As shown inFIGS. 5 and 7, in the illustrated embodiment, the primary combustion airoutlet 76 comprises a plurality of openings (also indicated by 76)spaced along the length of the rear side of the front duct 74. Theprimary combustion air feeds primary combustion of the fuel in thecombustion chamber 50. Preheating the primary combustion air results ina more complete and cleaner primary burn of the fuel. Because theprimary combustion air outlet 76 is positioned at a front of thecombustion chamber 50, the fuel burns locally, beginning at the front ofthe combustion chamber and progressing to the rear of the combustionchamber. The fuel (e.g., wood logs) burning locally from the front tothe back can be referred to as burning like a cigar. The lack of primarycombustion air outlets other places than adjacent the front end of thecombustion chamber cause the fuel to burn in this way. Ashes resultingfrom the burning fuel fall to the ash chamber 52. Other products ofcombustion, including heat, gases, and particulates, naturally rise inthe combustion chamber 50.

To achieve a complete burn of the fuel, secondary combustion air isdelivered to an upper portion of the combustion chamber 50 via thesecondary combustion air passage 64. The secondary combustion airpassage 64 includes a duct 80 extending downstream from a valve system81. The duct 80 extends forward from the valve system 81 to the front ofthe combustion chamber 50. The duct 80 is exposed inside the combustionchamber 50 to preheat secondary combustion air in the duct. Thepreheated secondary combustion air is emitted to the combustion chamber50 through a series of secondary combustion air openings 82 (referred tocollectively as an “outlet”). Secondary combustion air traveling in thesecondary combustion air passage 64 may be preheated to about 500degrees F. before reaching the most distal opening 82. In theillustrated embodiment, the secondary combustion air openings 82 arespaced along each side of the duct 80 and along the bottom of the ductfrom the rear to the front of the combustion chamber 50. Desirably, theopenings 82 are arranged to deliver a relatively uniform distribution ofsecondary combustion air along the length of the combustion chamber 50between the front and rear of the combustion chamber. The openings 82increase in size along the length of the duct 80 from the proximal(rear) end to the distal (forward) end of the duct. The openings 82 inthe proximal half at the sides and bottom of the duct are about 0.25inch in diameter, and the openings in the distal half at the sides andbottom of the duct are about 0.375 inch in diameter. The arrangement ofthe openings 82 and their diameters were determined by trial and errortesting, to provide the most efficient burn characteristics. Otherarrangements and sizes can be used without departing from the scope ofthe present disclosure. Desirably, the secondary combustion air causescombustible products remaining after primary combustion (e.g., carbonmonoxide) to combust before exiting the combustion chamber 50. Thepreheated secondary combustion air assists in achieving a bettersecondary combustion, and the secondary combustion generally assists inachieving a cleaner, more complete burn of the fuel before the productsof combustion leave the combustion chamber 50.

The combustion chamber 50 includes an exit 86 (FIG. 7) through whichproducts of combustion exit the combustion chamber. The exit 86 ispositioned adjacent the front of the combustion chamber 50 at the upperend of the combustion chamber, at the front end of the secondarycombustion air duct 80. In the illustrated embodiment, the exit 86 is arelatively short duct having a width extending between the oppositesides of the fire box 40. Air exits the combustion chamber 50 via theexit 86 on the left and right sides of the secondary combustion air duct80.

The configuration of the combustion air delivery system 60 and thearrangement of the outlets 76, 82 with respect to the exit 86 of thecombustion chamber 50 is designed to provide a longer residence time forproducts of combustion in the combustion chamber and thus more time forsecondary combustion to achieve a more complete burn. As illustrated byarrows in FIG. 7, primary combustion air is directed toward the rear ofthe combustion chamber 50. The primary combustion air outlet 76 isoriented to promote such air flow. The openings 76 on the rear wall ofthe duct 74 face rearward. As the fuel burns locally from front to rear,the products of combustion accumulate at a rear upper end of thecombustion chamber 50. As the products of combustion move forward towardthe combustion chamber exit 86, the products of combustion travel alongthe length of the secondary combustion outlet 82, subjecting theproducts of combustion to secondary combustion air for an extended time.Optimally, complete combustion is achieved by the time the products ofcombustion exit the combustion chamber 50.

In an aspect of the present disclosure, the combustion air deliverysystem 60 is configured to deliver variable flow of secondary combustionair to the combustion chamber 50. As will become apparent, thecombustion air delivery system 60 is configured to independently controlthe amounts of primary and secondary combustion air delivered to thecombustion chamber 50. As will be described in further detail below,ambient air supply to the secondary combustion air duct 80 is controlledby the valve system 81, which is actuated in response to temperaturechange. Air moves along the secondary combustion air duct 80 via naturaldraft. Regarding the primary combustion air, the draft blower 44 may beautomatically cycled between a forced draft mode in which the draftblower is energized and actively forces air through the primarycombustion air outlet 76, and a natural draft mode in which the blower44 is de-energized but permits air to flow by natural draft through theprimary combustion air outlet.

It has been determined that the amount of secondary combustion airshould change based on temperature in the combustion chamber 50 toachieve an optimum level of secondary combustion. In one example, theamount of secondary combustion air required is about the same at a givencombustion chamber temperature notwithstanding whether the draft blower42 is energized or de-energized. As will become apparent, the combustionair delivery system 60, and in particular the valve system 81, isconfigured to provide combustion air in this fashion.

The valve system 81 will now be described in further detail withreference to FIGS. 5-9. The valve system 81 generally includes a valvehousing 87, a valve member 88, and a temperature responsive valveactuator 89. In the illustrated embodiment, the valve housing 87 has afront outlet 87A mounted over the intake of the secondary combustion airpassage 80 and has a bottom inlet 87B (FIGS. 8, 9) to permit flow ofambient air into the housing. The valve member 88 is provided in theform of a flap pivotally mounted on the valve housing 87 by a pin 88A.The flap 88 has two pilot openings 88B permitting ambient air to enterthe valve housing 87 through the flap 88, for reasons explained below.The flap 88 is biased by gravity to rest on a lower wall of the valvehousing 87 in a position covering the inlet 87B of the valve housing. Asshown in FIG. 6, the temperature responsive valve actuator 89 includes abi-metal member 89A and a chain 89B (broadly, a link) connecting thebi-metal member to the flap 88. The bi-metal member 89A has the shape ofa coil and is sometimes referred to as a bi-metal spring or bi-metalcoil. The bi-metal member 89A includes strips of two different types ofmetals, such as steel and copper, joined together along their lengthsuch as by brazing or welding. It will be appreciated that the bi-metalmember 89A itself is a temperature sensor and an actuator. The bi-metalmember 89A converts temperature change into mechanical displacementwithout need for an electronic control. Because of the differentexpansion properties of the metals of the bi-metal member 89A, thebi-metal member tends to deflect by uncoiling in response to increase intemperature. As a result, the bi-metal member 89A actuates the valvesystem 81 by pulling the chain 89B to cause the flap 88 to pivot from aclosed position (e.g., as shown in FIGS. 5, 6, 8) to an open position(e.g., as shown in FIG. 9). If the bi-metal member 89A encounters adecrease in temperature, the bi-metal member tends to recoil and permitsgravity to move the flap 88 toward the closed position. The bi-metalmember 89A continuously monitors temperature and adjusts the position ofthe flap 88 by raising the flap in response to temperature increase andlowering the flap in response to temperature decrease. As a result, theamount of secondary combustion air delivered to the combustion chamber50 changes based on temperature sensed by the bi-metal member 89A.

The temperature responsive valve actuator 89 is configured to indirectlysense temperature of the combustion chamber 50. More specifically, thetemperature responsive valve actuator 89 is configured to indirectlysense temperature of the combustion chamber via radiation from the rearwall 30 of the housing 22 to which the valve housing 87 is mounted. Thetemperature responsive valve actuator 89 is also configured toindirectly sense temperature of the combustion chamber via conductionfrom the rear wall 30 of the housing through the valve housing 87 andsupport structure mounting the actuator to the valve housing (e.g., afastener such as a screw or bolt). Alternatively, the temperatureresponsive valve actuator 89 could be configured to directly orindirectly sense the temperature of products of combustion downstreamfrom the combustion chamber 50. For example, a sensor could bepositioned in or on an exterior of a passage downstream from thecombustion chamber 50. The illustrated bi-metal member 89A is outsidethe combustion chamber 50 and positioned to be out of the downstreamflow of products of combustion from the combustion chamber 50. Becausethe bi-metal member 89A indirectly senses temperature of the combustionchamber 50, the bi-metal member actuates the valve member 88 lesserratically. Directly sensing the temperature of the combustion chamber50 might lead to sporadic actuation of the valve system 81 because thecombustion chamber can experience relatively sharp temperature changespikes, such as when the draft blower 44 becomes energized. The indirectsensing provides a smoother valve actuation for less erratic change ofsecondary combustion air delivery.

A curve representing open flow area through the valve inlet 87B as afunction of temperature sensed by the bi-metal member 89A is shown inFIG. 13. A table including values used to populate the graph is providedbelow. The Valve Flow Area values are estimated to represent the degreeof openness of the valve 81 based on the position of the flap 88. Itwill be appreciated that when the draft bower 44 is energized, thetemperature in the combustion chamber 50 will increase, and in responsethere will be increased secondary combustion air due to the increase invalve flow area. When the draft blower 44 is not energized, thetemperature in the combustion chamber 50 will tend to decrease, and inresponse there will be decreased secondary combustion air due to thedecrease in valve flow area. In general, an increase in temperature inthe combustion chamber 50 is associated with increased products ofcombustion, which require increased secondary combustion air for acomplete burn. As shown in the table below and the graph of FIG. 13,desirably there is still some valve flow area (e.g., about 0.2 squareinches corresponding to the flow area of the pilot holes 88B in the flap88 ) such that secondary combustion air is provided at a low level evenwhen the temperature is low. This configuration is helpful for achievinga more complete burn (with secondary combustion air) when starting upthe furnace 20 from a cold condition.

Temp (° F.) Valve Flow Area (in²) 75 0.221 150 0.500 200 0.950 250 1.900300 3.400 350 4.600 400 5.250 450 5.750 500 6.000 550 6.000 600 6.000650 6.000 700 6.000

Accordingly, the combustion air delivery system 60 is configured suchthat, at the same combustion chamber temperature, the amount ofsecondary combustion air delivered to the combustion chamber 50 in thenatural draft mode is about the same as the amount of secondarycombustion air delivered to the combustion chamber in the forced draftmode. The amount of secondary air provided via the valve system 81results in the most complete burn for the particular construction of thefurnace. It will be appreciated that the amount of secondary combustionair needed to achieve a complete burn may vary by furnace design.

There are distinct advantages to achieving the desired amount ofsecondary combustion air and the desired ratio of secondary to primarycombustion air by the structural design of the combustion air deliverysystem 60. In the illustrated embodiment, it is not necessary to provideelectronic controls for adjusting the amount of secondary combustion airor tuning the secondary combustion air with respect to the primarycombustion air. Achieving the desired level of secondary combustion airwithout electronic control increases furnace durability and reliabilityfor the long term. The desired levels of primary and secondarycombustion air are fixed and set at the factory, such as by the designof the valve system 81 and structural design of the various passages andinlets/outlets. Ease of use for the consumer is improved and maintenanceis reduced because there are fewer electronically controlled componentsand fewer moving parts. In the event of a power failure, control ofsecondary combustion air is not lost (and emissions reduction ismaintained) because the valve system 81 is not electronicallycontrolled. Moreover, compared to a catalytic emissions reductionsystem, the valve system 81 is more effective at emissions reduction atlower combustion chamber temperature (e.g., when the draft blower 44 isidle), is less prone to blockage, and requires less maintenance.However, in some embodiments, an electronic control and/or a catalyticsystem can be used.

It will be understood that other combustion air delivery systems can beused without departing from the scope of the present disclosure. Thevarious components can have other forms, and components can be omitted.For example, the primary combustion supply air plenum 32 and the primaryand secondary combustion air passages 62, 64 may have otherconfigurations. The primary and secondary combustion air outlets 76, 82could include more or fewer openings (e.g., one), and the plenum inlet66 could include more than one opening. Moreover, the valve system 81could have other forms. It will be appreciated that the illustratedvalve system 81 is shown by way of example and not limitation. Forexample, other types or arrangements of valve systems could be usedincluding systems having other types of valve members (e.g., valvemembers made of multiple components and/or having more complex shapes).It will be appreciated that the bi-metal member 89A disclosed herein isboth a sensor for sensing temperature and an actuator for moving thevalve member 88. However, other temperature responsive valve actuatorshaving physically separate temperature sensors and actuators (e.g., withthe sensor positioned locally or remotely with respect to the actuator)can be used. Moreover, electronic controls can be used.

Another aspect of the furnace 20 that assists in achieving an efficientburn in the combustion chamber 50 is insulation 90 provided around thecombustion chamber 50. As shown in FIGS. 7 and 10, insulation 90 isprovided around the combustion chamber 50. For example, the insulationmay comprise vermiculite, fire bricks, calcium silicate, and/or steel.One type of steel that can be used is stainless steel, which canwithstand extreme combustion temperatures, provides good corrosionresistance, and maintains heat in the combustion chamber. The insulation90 contains heat in the combustion chamber 50, and a hotter combustionchamber results in cleaner, more complete combustion. Accordingly, fewerharmful emissions are emitted from the furnace.

Referring to FIGS. 10 and 11, the heat transfer device 42 is positionedabove the combustion chamber 50 and includes a front adjacent the frontof the housing 22, a rear adjacent the rear of the housing, and oppositesides adjacent the sides of the housing. The heat transfer device 42 isconfigured to efficiently transfer heat from the products of combustionto air delivered to the space for heating. The heat transfer device 42is positioned downstream from the combustion chamber 50 so that heat istransferred from the products of combustion after a substantiallycomplete burn in the combustion chamber. The heat transfer device 42includes a post-combustion plenum 100 having an inlet 102 in fluidcommunication with the combustion chamber 50 for receiving products ofcombustion after combustion has occurred in the combustion chamber. Thepost-combustion plenum 100 has an exit 104 for permitting products ofcombustion to exit the plenum once heat is transferred to the air beingdelivered to the space being heated. Heat transfer passaging 106 extendsaround the post-combustion plenum 100 for transferring heat from theproducts of combustion to air to be heated, as will be described infurther detail below.

The furnace 20 communicates with the space to be heated via a ductsystem, a portion of which is illustrated in FIG. 7. The duct systemincludes a cool air duct 110A (e.g., a cool air return), which suppliescool air to the heat transfer device 42, and at least one heated airduct 110B, which transports heated air to the space being heated. Thefurnace 20 has a forced-air system 120 that forces relatively cool airfrom the cool air duct 110A through the heat transfer device 42 and outthrough the heated air duct 110B. Duct systems having otherconfigurations such as duct systems without a cool air duct, and systemshaving air inlets and outlets of other sizes and shapes can be used.

The forced-air system 120 moves air from the cool air duct 110A throughthe heat transfer device 42 to the heated air duct 110B. The system 120includes the blower 46, a cool air plenum 122, and two ducts 124upstream from the heat transfer device 42. Flow of air through theforced-air system 120 is illustrated by arrows in FIGS. 10 and 11. Thecool air plenum 122 is in fluid communication with the blower housing 34through a rectangular inlet 128 (e.g., having a flow area of about 60square inches). The blower 46 blows air from the blower housing 34through the inlet 128 into the cool air plenum 122 below the fire box40. In the illustrated embodiment, the cool air plenum 122 has a lengthextending between the front and rear walls 28, 30 and a width extendingbetween the left and right walls 36, 38. The cool air plenum 122 is influid communication with two ducts 124 positioned on the left and rightsides of the firebox 40. The ducts 124 are defined by spaces between thefirebox 40 and the left and right housing walls 36, 38. Vanes 130 in theducts 124 (FIG. 11) direct air rearward in the ducts 124 as the airrises. The insulation 90 mounted on the sides of the combustion chamber50 insulates the ducts 124 from the combustion chamber, so thecombustion chamber is not cooled by the air. The ducts 124 lead toopposite sides of the heat transfer device 42.

The heat transfer device 42 will now be described in further detail withreference to FIG. 11. The heat transfer device 42 is configured toachieve efficient heat transfer from the products of combustion to theair flowing through the heat transfer passaging 106. The post-combustionplenum 100 directs the products of combustion along the length of theplenum from the inlet 102 adjacent the front end of the plenum to theexit 104 adjacent the rear end of the plenum. The post-combustion plenum100 includes a bottom wall dividing the combustion chamber 50 from theplenum, side walls extending up from the bottom wall, and a top wallspanning the side walls. The construction provides the plenum 100 with agenerally rectangular cross section. The post-combustion plenum 100 hassix sides, namely a front side, a rear side, a top side, a bottom side,a left side, and a right side. The post-combustion plenum 100 has alength extending between the front and rear sides, a width extendingbetween the left and right sides, and a height extending between the topand bottom sides. As the products of combustion flow through the plenum100, they flow along the side and top walls of the plenum from the frontside of the plenum toward the rear side of the plenum. Downstream fromthe heat transfer device 42, the products of combustion exit the housing22 via the exit 104 in the rear wall 30 of the housing 22, to which achimney (not shown) may be connected.

The heat transfer passaging 106 of the heat transfer device 42 includesseveral passage portions extending along the post-combustion plenum. Asdescribed in further detail below, the heat transfer passaging 106includes two side passage portions 106A, a forward passage portion 106B,and an upper passage portion 106C. In the illustrated embodiment, theheat transfer passaging 106 extends across substantially all of the top,left, and right sides of the post-combustion plenum 100 (three of thesix sides) for efficient heat transfer over a large surface area. Thetwo side passage portions 106A (broadly, first passage portions) are onthe respective left and right sides of the post-combustion plenum 100and extend lengthwise along the left and right sides toward the frontside. The side passage portions 106A receive air from the respectiveducts 124. Air in the side passage portions 106A flows forward towardthe front side of the post-combustion plenum. The forward passageportion 106B (broadly, third passage portion) is on the front side ofthe post-combustion plenum 100 and extends widthwise along the frontside. The forward passage portion 106B is downstream from and receivesair from both of the side passage portions 106A. Air in the forwardpassage portion 106B flows generally widthwise along the front side ofthe post-combustion plenum 100, inward from the opposite side passageportions 106A toward a middle of the forward passage portion. The upperpassage portion 106C (broadly, second passage portion) is on the topside of the post-combustion plenum 100 and extends lengthwise along thetop side toward the rear side. Air in the upper passage portion 106Cflows toward the rear side of the post-combustion plenum. Ultimately,the heated air exits the upper passage portion through one or more exits140 (FIG. 1). In the illustrated example, two exits 140 in the form ofcircular openings (e.g., each having an 8 inch diameter) are provided inthe upper wall 24 of the housing 22 to which the ducts 110B can beconnected. Routing the heat transfer passaging 106 along substantiallyall of three sides of six sides of the post-combustion plenum 100provides increased efficiency in heat transfer from the products ofcombustion to the air passing through the heat transfer passaging 106.For example, air entering the heat transfer passaging 106 may have atemperature of about 75 degrees F., and air exiting the heat transferpassaging may have a temperature of about 230 degrees F.

The forced-air system 120 and the heat transfer passaging 106 areconfigured to provide a generally constant speed air flow through thefurnace 20. This has been found to improve efficiency of heat transfer.For example, the side ducts 124 are sized to have a cross-sectional flowarea (e.g., 30 square inches) about half the flow area of the inlet 128(about 60 square inches). The air flowing upward along the ducts 124 isinsulated from the combustion chamber 50 so the air flow speed does notincrease significantly. As shown in FIG. 11, the side passage portions106A have side wall partitions 144 shared with the upper passage portion106C. The side wall partitions 144 taper inward toward the front side ofthe post-combustion plenum. The construction is such that the side wallpartitions 144 provide the side passage portions 106A with increasingdownstream flow area along the flow path of the side passage portions.The key-shaped exits of the side passage portions (FIG. 11) have greaterflow areas (e.g., about 30.1 square inches, totaling about 60.2 squareinches) than the side ducts 124. Heat transfer occurs in the sidepassage portions 106A, so the increase in flow area assists inmaintaining a generally constant speed air flow. The exit of the forwardpassage portion 106B has a slightly greater flow area (about 60.3 squareinches). The side wall partitions 144 provide the upper passage portion106C with increasing downstream flow area along the rearward flow pathof the upper passage portion. The exits 140 (FIG. 1) from the upperpassage portion 106C have a combined flow area (e.g., about 100.5 squareinches), to provide increased flow area. It will be appreciated that asair in the heat transfer passaging 106 is heated, the air will tend toflow faster. However, the increasing downstream flow area of the heattransfer passaging 106 assists in providing a generally constant speedair flow through the heat transfer passaging, which has been found toprovide increased efficiency in heat transfer. In total, the forced-airblower path through the furnace 20 is constructed to providequasi-constant low speeds, to optimize heat transfer, while not removingheat in areas (e.g., the ducts 124) that would compromise efficiency ofcombustion in the combustion chamber 50.

The furnace 20 includes various electrical components for controllingoperation of the furnace. In particular, the furnace 20 includes a mainelectrical control 150 and a blower limit switch 152. The mainelectrical control 150 is mounted on the control housing 35 and isconnectable to a power source such as an electrical outlet or generator.The main electrical control 150 is adapted for communication with athermostat (not shown) located within the space to be heated. The blowerlimit switch 152 (FIG. 4) includes a probe 152A (FIGS. 10, 11)positioned in the right side passage portion. The blower limit switch152 senses temperature within the right side passage portion 106A viathe probe 152A and communicates this information to the main electricalcontrol 150 so that the main electrical control can selectively energizeand de-energize the blower 46.

In operation, a fuel source such as wood is loaded in the combustionchamber 50, and the fuel is ignited. When the thermostat in the space tobe heated calls for heat, the main electrical control 150 causes thedraft blower 44 to energize (forced draft mode), forcing oxygen out ofthe primary combustion air outlet 76 to feed the fire in the combustionchamber 50. As temperature rises in the combustion chamber 50, the valvesystem 81 will permit increased secondary combustion air flow to thecombustion chamber. When the blower limit switch 152 senses apreselected hot air temperature (e.g., 170° F.) within the right sidepassage portion 106A, the main electrical control 150 causes the blower46 to energize to force the heated air to the space being heated anddraw replacement air into the furnace. If the blower limit switch 152senses a preselected cool air temperature (e.g., 110° F.) within theheated air plenum 50B, the main electrical control 150 de-energizes theblower 46 until the blower limit switch 152 again senses the preselectedhot air temperature in the right side passage portion 106A. When thethermostat senses sufficient heat in the space being heated, the mainelectrical control 150 de-energizes the draft blower 44, decreasingoxygen to the fire to decrease heated air generation in the furnace 20.However, combustion air is still delivered to the fire through theprimary combustion air outlet 76 by natural draft. As temperaturedecreases in the combustion chamber 50, the valve system 81 permitsdecreased secondary combustion air flow via the outlet 82.

The furnace 20 may be used as a sole source for heating the interior ofa building or a plurality of rooms of a building. The large size of thecombustion chamber 50, in combination with various other features of thefurnace described above, provide the furnace with the capability ofproviding a significantly large amount of heat with good efficiency andsignificantly lower emissions of particulates and carbon monoxide.

It will be appreciated various aspects of the furnace described hereincan be modified. For example, features can be omitted or have otherforms. Moreover, it will be appreciated that the dimensions noted hereinare provided by way of example and not as a limitation.

Having described the disclosure in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the appended claims.

When introducing elements of the present disclosure or the preferredembodiment(s) thereof, the articles “a”, “an”, “the”, and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including”, and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions, products,and methods without departing from the scope of the disclosure, it isintended that all matter contained in the above description and shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:
 1. A forced-air furnace for heating a space, the furnace comprising: a housing having a top, bottom, front, rear, and opposite sides; a firebox in the housing having a combustion chamber adapted for receiving and burning fuel to produce heated products of combustion; a combustion air delivery system for delivering combustion air to the combustion chamber, the combustion air delivery system including: a primary combustion air passage including a primary combustion air outlet in the combustion chamber for delivering primary combustion air to the combustion chamber at a primary combustion air flow rate, a secondary combustion air passage including a secondary combustion air outlet positioned above the primary combustion air outlet in the combustion chamber for delivering secondary combustion air to the combustion chamber at a secondary combustion air flow rate, and a valve system fluidly isolated from the first combustion air passage and in fluid communication with the secondary combustion air passage, said valve system being configured for changing the secondary combustion air flow rate delivered to the combustion chamber in response to a change in combustion chamber temperature without changing the primary combustion air flow rate delivered to the combustion chamber; and a heat transfer device above the firebox including a post-combustion plenum having an inlet in fluid communication with the combustion chamber for receiving products of combustion therefrom and an exit for permitting products of combustion to exit the post-combustion plenum, the post-combustion plenum having a first side, a second side opposite the first side, and a length extending therebetween, wherein the heat transfer device includes heat transfer passaging adapted for transporting air to be heated by the post-combustion plenum, the heat transfer passaging including a first passage portion extending lengthwise relative to the post-combustion plenum defining a flow path extending adjacent to the post-combustion plenum and configured for directing air in a direction toward the first side of the post-combustion plenum and a second passage portion downstream from the first passage portion extending lengthwise relative to the post-combustion plenum defining a flow path extending adjacent the post-combustion plenum and configured for directing air in a direction toward the second side of the post-combustion plenum.
 2. A forced-air furnace as set forth in claim 1, wherein the valve system is configured to increase the secondary combustion air flow rate delivered to the combustion chamber in response to an increase in combustion chamber temperature.
 3. A forced-air furnace as set forth in claim 2, wherein the valve system is configured to decrease the secondary combustion air flow rate delivered to the combustion chamber in response to a decrease in combustion chamber temperature.
 4. A forced-air furnace as set forth in claim 1, wherein the valve system is free of an electronic control for the valve.
 5. A forced-air furnace as set forth in claim 1, wherein the valve system includes a valve member and a temperature responsive valve actuator, the valve member being movable by the temperature responsive valve actuator in response to a change in combustion chamber temperature to change the secondary combustion air flow rate delivered to the combustion chamber.
 6. A forced-air furnace as set forth in claim 5, wherein the temperature responsive valve actuator comprises a bimetal member.
 7. A forced-air furnace as set forth in claim 5, wherein the temperature responsive valve actuator is located outside the combustion chamber thereby indirectly sensing the combustion chamber temperature.
 8. A forced-air furnace as set forth in claim 1, wherein the valve system comprises a valve member mounted for movement between an open position and a closed position, and the combustion air delivery system is configured to permit air to flow to the combustion chamber via the secondary combustion air passage when the valve member is in the closed position.
 9. A forced-air furnace as set forth in claim 1, wherein the combustion air delivery system is configured to independently control the primary and secondary combustion air flow rates delivered to the combustion chamber.
 10. A forced-air furnace as set forth in claim 1, wherein the combustion air delivery system is configured to draw secondary combustion air through the secondary combustion air passage via natural draft.
 11. A forced-air furnace as set forth in claim 1, wherein the heat transfer passaging includes a third passage portion at the first side of the post-combustion chamber, the third passage portion being in fluid communication with and downstream from the first passage portion and in fluid communication with and upstream from the second passage portion.
 12. A forced-air furnace as set forth in claim 1, wherein: said first side comprises a front side; said second side comprises a rear side; the post-combustion plenum has six sides including a top side, a bottom side, a left side, a right side, said front side, and said rear side; and the heat transfer passaging extends across substantially all of at least two of said six sides of the post-combustion plenum.
 13. A forced-air furnace for heating a space, the furnace comprising: a housing having a top, a bottom, a front, a rear, and opposite sides; a firebox in the housing having a combustion chamber adapted for receiving and burning fuel to produce heated products of combustion, the combustion chamber having a front face adjacent the front of the housing, a rear face opposite the front face, a top face above said rear face, a bottom face below said front face and said rear face, opposite side faces extending from the top face to the bottom face and from the front face to the rear face, and an exit adjacent the front face permitting heated products of combustion to exit the combustion chamber; a combustion air delivery system for delivering combustion air to the combustion chamber, the combustion air delivery system including: a primary combustion air passage including a primary combustion air outlet in the combustion chamber for delivering primary combustion air to the combustion chamber at a primary combustion air flow rate, and a secondary combustion air passage including a secondary combustion air outlet positioned above the primary combustion air outlet in the combustion chamber for delivering secondary combustion air to the combustion chamber at a secondary combustion air flow rate; and a heat transfer device above the firebox including: a post-combustion plenum having an inlet at an upstream end in fluid communication with the combustion chamber exit for receiving heated products of combustion from the combustion chamber and an outlet at a downstream end for permitting products of combustion to exit the post-combustion plenum, the post-combustion plenum having a thermally conductive top wall extending from the upstream end to the downstream end, and opposite thermally conductive side walls extending downward from the top wall and from the upstream end to the downstream end, and heat transfer passaging extending outside the post-combustion plenum adapted for transporting fluid past the post-combustion plenum, said heat transfer passaging including: first passage portions extending adjacent the opposite thermally conductive side walls of the post-combustion plenum, thereby heating fluid passing through the first passage portions, and a second passage portion downstream from the first passage portions extending adjacent the thermally conductive top wall of the post-combustion plenum, thereby heating the fluid passing through the second passage portion; and a valve system fluidly isolated from the first combustion air passage and in fluid communication with the secondary combustion air passage, said valve system being configured for changing the secondary combustion air flow rate delivered to the combustion chamber in response to a change in combustion chamber temperature without changing the primary combustion air flow rate delivered to the combustion chamber.
 14. A forced-air furnace as set forth in claim 13, wherein: said first passage portions extend adjacent the opposite thermally conductive side walls of the post-combustion plenum from an upstream end adjacent the downstream end of the post-combustion plenum to a downstream end adjacent the upstream end of the post-combustion plenum; and said second passage portion extends adjacent the thermally conductive top wall of the post-combustion plenum from an upstream end adjacent the upstream end of the post-combustion plenum to a downstream end adjacent the downstream end of the post-combustion plenum.
 15. A forced-air furnace as set forth in claim 13, wherein: the secondary combustion air passage directs air toward the front of the combustion chamber; a first outlet of said plurality of secondary combustion air passage outlets faces the bottom face of the combustion chamber to direct air passing through the second combustion air passage downward into the combustion chamber; and a second outlet and a third outlet of said plurality of secondary combustion air passage outlets face the opposite sides faces of the combustion chamber to direct air passing through the second combustion air passage laterally into the combustion chamber. 